3,4,5,3',4',5'-hexachlorobiphenyl (HCB) was fed to 1-day-old chicks at varying concentrations from 0.01 to 60.0 ppm in feed for 21 days. The LD50 appears to lie between 1 and 2 ppm of HCB in feed. All chicks died between 6 and 18 days fed HCB at a concentration of 3.0 ppm and more. A cumulative total daily intake of HCB from 0.12 mg to 0.32 mg at 3.0 ppm level in feed caused 100 percent mortality. Ascites, hydropericardium and subcutaneous edema developed in all chicks fed 3.0 ppm and above. Red fluorescence under UV light denoting abnormal accumulation of porphyrin was observed in the mucosal layer of gizzard, femur, sternum, skull, liver and cecum. Atrophy of bursa and thymus, necrosis and inflammatory changes in the mucosal layer of gizzard, and necrosis of hepatocytes were observed microscopically. 3,4,5,3',4',5'-HCB appears to be the most toxic isomer of PCB tested in avian species.